In semiconductor fabrication it is necessary to transport substrates from one station to another. For example, it is necessary to move substrates from a cassette station to a processing station and/or metrology tool. Typically, substrates are transported using a robotic arm.
FIG. 1 shows a conventional substrate processing environment 10. Processing environment 10 includes a stage 12, a load cassette station 14, an unload cassette station 16 and a robot transfer device 18. It should be understood that the stage 12 may alternatively be used with a metrology unit, a processing unit or a combination processing/metrology unit. As shown in FIG. 1, the robot transfer device 18 has a single arm 20 that is used to transport a substrate (not shown) from the load cassette station 14 to the stage 12 and from the stage 12 to the unload cassette station 16.
A disadvantage with conventional stage 12 is that it has a single substrate support and thus has a single substrate capacity. As a result, the robot transfer device 18 must first unload a processed substrate from the stage 12 and place the processed substrate in the unload cassette station 16 before the robot transfer device 18 can remove an unprocessed substrate from the load cassette station 14 and load the unprocessed substrate onto the stage 12. Thus, stage 12 is idle while the robot transfer device 18 unloads the processed substrate and loads the unprocessed substrate. The movements of the robot transfer device 18 take time and thus increase the idle time of the stage 12 and increase the amount of time required for substrate handling. As a result, the throughput, i.e., the number of substrates processed per unit of time, is lower than desirable.
Therefore, there is a need for a stage having an inexpensive and relatively simple buffering station for supporting one or more substrates to increase the throughput of the system.